1. Field of the Invention
This invention relates to an integrated semiconductor device comprising a light modulator of an electro-absorption type semiconductor and an optical device, such as a semiconductor laser, useful for an optical communication and/or optical data processing.
2. Description of the Related Art
An integrated EA-DFB device comprising an electro-absorption type (hereinafter “EA”) modulator and a distributed feed back type (hereinafter “DFB”) laser converts high-speed electrical signals to optical signals.
Continuous wave (hereinafter “CW”) light which is generated when electric current is applied to the DFB laser is input to the EA modulator through a butt joint section where the DFB laser and the EA modulator are joined. The butt joint section is an interface between an active layer of the DFB laser for oscillating a laser beam and a light absorption layer of the EA modulator. The EA modulator has an electro-absorption effect, wherein the amount of absorbed light is changed due to the change of the semiconductor band structure when a voltage is applied to the EA modulator. Accordingly, when a reverse bias voltage is applied to the EA modulator, the amount of absorbed light is increased so that light does not permeate through the light absorption layer of the EA modulator. When a voltage is not applied to the EA modulator, light permeates the EA modulator (ON condition) and, when a reverse voltage is applied, light is cut off (OFF condition), thereby to modulate light. The EA modulator converts electrical signals to optical signals by changing the light absorption rate by applying a voltage to the light absorption layer from outside.
FIGS. 4 and 5 show a conventional EA-DFB device. A DFB laser (hereinafter “LD”) 41 and an EA modulator 42 are formed on a substrate. A separation region 43 is provided between an upper electrode 46 of the LD 41 and an upper electrode 48 of the EA modulator 42. In the LD 41, a lower clad layer 53, an active layer 54, an upper clad layer (not shown), and an ohmic contact layer (not shown) to be brought into contact with the electrode are formed in this order on a substrate 52. Tn the EA modulator 42, the lower clad layer 53, a light absorption layer 56, the upper clad layer, and the ohmic contact layer are formed in this order on the substrate 52. Tn the separation region 43, the lower clad layer 53, a wave guide layer 55, the upper clad layer, and the ohmic contact layer are formed in this order on the substrate 52. An etched channel 49 is provided on sides of the LD 41 and the EA modulator 42 so as to form a ridge structure.
A forward bias voltage is applied between the upper electrode 46 and a lower electrode 47 of the LD 41 so as to generate CW light from the active layer 54. A reverse bias voltage is applied between the upper electrode 48 and the lower electrode 47 of the EA modulator 42 so as to change the amount of light absorbed by the absorption layer 56 for performing modulation. The contact layer under a pad of the upper electrode 48 is etched so as to prevent deterioration of the frequency characteristics caused by the increased electric capacity of the pad electrode.
However, in this conventional EA-DFB device, when input light is intense, the vicinity of the incident area of the EA modulator 42, where the amount of absorbed light is the largest, is adversely affected by the heat generation by the absorption of light. Accordingly, there has been a problem that intense light cannot be input.
FIG. 6 shows an EA-DFB device proposed by Japanese Patent Application Kokai Number 2001-117058 to solve the above-mentioned problem. In the same manner as the conventional art shown in FIG. 5, an LD 61 and an EA modulator 62 are formed on a substrate, and a separation region 63 is provided between a upper electrode 66 of the LD 61 and an upper electrode 68 of the EA modulator 62. In the LD 61, a lower clad layer 73, an active layer 74, an upper clad layer (not shown), and an ohmic contact layer (not shown) to be brought into contact with the electrode are formed in this order on a substrate 72. In the EA modulator 62, the lower clad layer 73, a light absorption layer 76, the upper clad layer, and the ohmic contact layer are formed in this order on the substrate 52. In the separation region 63, the lower clad layer 73, a wave guide layer 75, the upper clad layer, and ohmic contact layer are formed in this order on the substrate 72. An etched channel 79 is provided on sides of the LD 61 and the EA modulator 62.
The EA-DFB device shown in FIG. 6, however, is provided with a region 64, where the upper clad layer is extended such that the channel 79 in the vicinity of the incident area of the EA modulator 62 is made narrower (the separation 63 region is made wider) . This structure improves the heat-radiation property in the vicinity of the incident area where the rise of temperature is highest, thus enabling the input of more intense light.
However, the above EA-DFB device has a problem that the EA modulator and the LD are not completely separated electrically so that when a forward bias of +1.2 V is applied to the LD and a reverse bias of −3 V is applied to the EA modulator, the separation region between the EA modulator and the LD has a strong electric field. Consequently, the absorption of light is not performed in the EA modulator but in the separation region so that the heat generated in the separation region by photo-current is not radiated efficiently in the region 64 in the vicinity of incident area of the EA.
FIG. 7 shows the principle of heat generation in the above EA-DFB device. A forward bias of +1.2 V is applied between the upper and lower electrodes 66 and 67 of the LD 61 to emit CW light from the active layer 64 and a reverse bias of −3 V is applied between the upper and lower electrodes 68 and 67 of the EA modulator 62 to change the amount of light absorbed in the absorption layer 65. Under this condition, a high voltage is applied between the upper electrodes 66 and 68 so that a light absorption 69 is performed in the separation region 63 resulting in the increased heat generation in the separation region 63.
FIG. 8 shows the result of simulation for relationship between the positions of the LD, the separation region, and the EA modulator, and photo-current generated by the light absorption. The simulation shows that the photocurrent in the separation region is highest, which means that the heat-generation in the separation region is highest.
Also, a slab under the pad electrode of the EA modulator increases the electric capacity of the entire pad electrode, which adversely affects the frequency characteristics.